SRS ANSWER TIME
If you have any doubt whatsoever, delete it / don't watch. If you are in the U.S., you will get in trouble for it if the authorities can prove that it was kiddie porn. Proving that it was porn is going to be easy. Proving that it was a kiddie might be harder.
If you end up with some skanky movie of an anonymous 17 year and 11 month old, is a prosecutor going to be able to put you in jail for the rest of your natural life? Dunno. If I was on a jury, I'd probably have reasonable doubt about the age in that situation. But if you have lots of tapes of obvious 13 year olds on your hard drive,